Heat insulation material placed in external walls, ceilings, roofs, floors and other areas about a building typically comprise fibrous blanket insulation, such as elongated blankets formed of fiberglass. A principle of the blanket insulation is to form dead air spaces that provide insulation against convection and conduction heat transfer to and from the heat controlled interior spaces of the building structure. The blanket insulation can be formed in small “clumps” and blown into spaces such as into the attics of residential homes and other areas about building structures, and also can be made into elongated blankets formed in a specific width and thickness that are suitable for placement between parallel joists, studs, rafters, purlins and other parallel support structures that are uniformly spaced apart. The elongated blanket, such as a fiberglass blanket, usually is supplied in reels and is cut to the desired length at the job site for placement between the parallel structures.
An example of heat insulation material is fiberglass that is one of the more desirable materials for forming blanket insulation because it holds its shape and traps a substantial amount of air between its fibers to form the dead air spaces. However, the fiberglass alone usually does not provide adequate heat insulation against radiant heat transfer.
A sheet of radiant heat reflective material has been applied in building structures, sometimes in combination with other materials such as fibrous blanket material. The reflective material, such as aluminum foil, provides a reflective surface for reflecting radiant heat, thereby functioning as a barrier to radiant heat transfer, and enhancing the insulation capabilities of the other heat insulation materials.
Another insulation innovation that has been developed is the use of phase change material (“PCM”) in combination with other heat insulation materials. The PCM loses heat when it changes phase from a liquid to a solid and absorbs heat when it changes phase from a solid to a liquid. These changes of phase occur at a substantially constant temperature for the PCM. The net result is that when the PCM is used in a wall structure, such as an external wall structure, and the temperature of the outside surface of the wall structure begins to rise from a temperature lower than the phase change temperature to a temperature higher than the phase change temperature, and the rising heat is transferred to the PCM, the PCM will remain at its phase change temperature as the PCM changes phase from a solid to a liquid. As it changes phase, the PCM absorbs heat transferred from the outside surface of the wall structure without changing its own temperature. This effectively delays the transfer of heat from the outside surface of the external wall to the inside of the building structure, reducing the load to be carried by the conventional air conditioning system of the building structure.
The reverse is true when the outside surface temperature of the wall structure becomes lower than the phase change temperature of the PCM. The PCM changes phase from liquid to solid at a substantially constant temperature, gradually giving up its heat to the outside cooler atmosphere. This delays the transfer of heat from the warmer interior of the building to the cooler outside atmosphere.
The use of PCM as an insulator for building structures is disclosed in U.S. Pat. Nos. 5,626,936 and 6,645,598 and U.S. published Patent Application 2008/0282637, published Nov. 20, 2008, which are incorporated herein by reference.
Although the use of PCM has been disclosed in the prior art as being used as an insulator for building structures, there may be times when the temperature transmitted through one surface of a wall to the PCM located in the wall does not pass the phase change temperature of the PCM. This makes the PCM ineffective to reduce the transfer of heat through the outside wall by changing phase. For example, when the PCM is placed in an exterior wall, the temperature of the outside surface of the exterior wall as heated by the sun in day time hours is the source of the heat that is transferred through the wall to the PCM. The day time temperature of the outside surface of the exterior wall facing the sun is likely to be significantly higher than the atmospheric temperature. For example, the outside surface of a sun-heated external wall may be 50° F. higher than the atmospheric temperature. At night time when the sun is not available to heat the exterior wall, the outside surface of the exterior wall may cool but not pass the phase change temperature of the PCM. This makes the PCM substantially ineffective to insulate the wall structure. A similar problem may be encountered in the cold season when the outside temperature of the external wall remains so cold that it is not able to change the PCM into its liquid state.
For example a PCM in an outside wall might be selected that changes phase at a temperature of 70° F. which is just below the controlled interior temperature of 74° F. of the building structure. The PCM would begin to change from a liquid to a solid and give up heat to the outside of the building structure as the night time outside temperature drops below 70° F. This delays the transfer of heat from the heat controlled interior of the building. However, if the temperature of the outside surface of the external wall in the warm season does not decrease enough to pass the phase change temperature of the PCM, the PCM does not change phase from liquid to solid and therefore does not have the capacity to change from a solid back to a liquid the next day and insulate the wall structure from the heat of the next day. Likewise, if the temperature of the outside surface of the external wall in the cold season does not increase enough in the day time hours to pass the phase change temperature of the PCM, the PCM does not change phase from solid to liquid and therefore during the next day there is no liquid PCM available to change back from a liquid to a solid and insulate the wall structure from the heat of the outside surface of the wall structure.
While the foregoing descriptions of how single PCMs may work in an outside wall structure of a building, there are other situations where similar uses of single PCMs would not perform to change phase and therefore not function efficiently to retard the transfer of heat. For example, PCMs may be used in interior walls of a building structure in which a heat source is present on one side of the internal wall and the heat source is intermittently operated to produce heat, such as a timed thermostat of the periodic use of a stove. The heat generated in these situations could function to change the phase of a PCM in the internal wall to a liquid. When the heat source in the room is turned off and the temperature in the room begins to drop below the phase change temperature of the PCM, the PCM could begin to change phase from a liquid to a solid. This would maintain the temperature of the room at a desired level until the PCM changes phase. However, if the heat source in the room does not raise the temperature of the PCM in the wall to a level higher than its phase change temperature, the system of preserving the room temperature does not work.
Thus, there is a need to provide a PCM-insulated wall structures and a method of insulating such wall structures that insulates at both high and low temperatures of the surfaces of the wall structures.